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Why Your 500GB Drive Shows 465GB (Binary vs Decimal)

July 5, 2025 โ€ข 4 min read

You buy a 500 GB hard drive, plug it in, and your computer says it only has 465 GB. Are you being cheated? No โ€” this is the result of two different ways of counting bytes. Here is why it happens and the maths behind it.

The Two Counting Systems

Decimal: 1 GB = 1,000,000,000 bytes (10^9)

Binary: 1 GiB = 1,073,741,824 bytes (2^30)

Difference: about 7.4% per unit

Why the Mismatch Exists

  • โ–ถ Drive manufacturers use decimal (base-10) definitions because they are industry standard and give larger numbers
  • โ–ถ Operating systems (especially Windows) historically use binary (base-2) definitions but still label them "GB"
  • โ–ถ A 500 GB drive contains exactly 500,000,000,000 bytes as advertised
  • โ–ถ Windows divides by 1,073,741,824 (binary) instead of 1,000,000,000 (decimal)
  • โ–ถ 500,000,000,000 รท 1,073,741,824 = 465.66 GiB, displayed as "465 GB"

How Much "Missing" Space by Drive Size

  • โ–ถ 128 GB drive: Shows as ~119 GB (9 GB "missing")
  • โ–ถ 256 GB drive: Shows as ~238 GB (18 GB "missing")
  • โ–ถ 500 GB drive: Shows as ~465 GB (35 GB "missing")
  • โ–ถ 1 TB drive: Shows as ~931 GB (69 GB "missing")
  • โ–ถ 2 TB drive: Shows as ~1.81 TB (190 GB "missing")
  • โ–ถ 4 TB drive: Shows as ~3.63 TB (370 GB "missing")

The larger the drive, the bigger the apparent discrepancy in absolute terms.

The Proper Names (That Nobody Uses)

The IEC created unambiguous terms in 1998:

  • โ–ถ Kibibyte (KiB): 1,024 bytes (binary kilobyte)
  • โ–ถ Mebibyte (MiB): 1,048,576 bytes (binary megabyte)
  • โ–ถ Gibibyte (GiB): 1,073,741,824 bytes (binary gigabyte)
  • โ–ถ Tebibyte (TiB): 1,099,511,627,776 bytes (binary terabyte)

macOS switched to decimal (base-10) reporting in 2009, so Macs show drive sizes matching the label. Windows still uses binary calculations. Linux varies by distribution.

Other Reasons for Less Usable Space

  • โ–ถ File system overhead: The formatting structure itself takes some space (1-5%)
  • โ–ถ Hidden partitions: Recovery partitions, system files, and EFI partitions
  • โ–ถ Pre-installed software: Some drives come with backup utilities

So between binary/decimal confusion and system overhead, your usable space is always less than the number on the box โ€” but you are getting exactly what was advertised in terms of actual bytes.

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